1. Field of the Invention
This invention relates generally to apparatus for indicating pressure in excess of a predetermined value. In particular, this invention relates to apparatus for one-time use for indicating pressure in excess of a predetermined value.
2. Description of Related Art
Injection molding is a process for molding wherein material to be molded is conditioned to be flowable, the flowable material, hereinafter referred to as melt, is forced into mold cavities and allowed to solidify, and the solid articles are removed from the mold cavities. To achieve complete filling of mold cavities the melt is subjected to high forces, resulting in high pressure within the melt. Equipment for controlling the flow of melt to the mold cavities can be damaged by excessively high melt pressure. Damage may not be evident from the exterior of the equipment, and the first indication of damage may appear as defects in articles being molded. As a measure of quality control, equipment returned for repair or replacement is assessed for defects in material and workmanship. To reduce the cost of such assessments, it is desirable to know whether failed equipment has been subjected to conditions outside the specified operating range. It is therefore desirable to provide a pressure indicator for injection molding equipment that can provide an indication that the equipment has been subjected to excessive internal pressure.
Pressure indicating devices for use with pressurized fluids are known wherein an indicating member is retained in a reset position within a fitting by magnetic force and is driven to an indicating position when a sensed pressure produces sufficient force to overcome the magnetic force. In such indicating apparatus, the indicating member is driven to the indicating position by biasing means which maintain the indicating member in the indicating position even when sensed pressure is relieved. Such devices are known for use in conjunction with filters, the triggering of the indicator occurring when, due to contamination, a filter fails to pass a sufficient volume of fluid. With the filter replaced, the indicating member can be restored to the reset position where it will remain until sensed pressure is again sufficient to overcome the magnetic force. Because of the potential for damage to injection molding equipment from excessive melt pressure, it is desired that a pressure indicator not be restored to a normal condition except in connection with a thorough examination and/or repair or reconditioning of the equipment. Hence there remains a need for pressure indicating apparatus that is intended to continue to indicate an occurrence of excessive pressure until the indicating device is remanufactured or replaced.